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+.. _kea-lfc:
+
+***************
+The LFC Process
+***************
+
+.. _kea-lfc-overview:
+
+Overview
+========
+
+``kea-lfc`` is a service process that removes redundant information from
+the files used to provide persistent storage for the memfile database
+backend. This service is written to run as a standalone process.
+
+While ``kea-lfc`` can be started externally, there is usually no need to
+do so. ``kea-lfc`` is run on a periodic basis by the Kea DHCP servers.
+
+The process operates on a set of files, using them to receive input and
+output of the lease entries and to indicate what stage the process is
+in, in the event of an interruption. Currently the caller must supply
+names for all of the files.
+
+.. _kea-lfc-usage:
+
+Command-Line Options
+====================
+
+``kea-lfc`` is run as follows:
+
+::
+
+   kea-lfc [-4 | -6] -c config-file -p pid-file -x previous-file -i copy-file -o output-file -f finish-file
+
+The argument ``-4`` or ``-6`` selects the protocol version of the lease
+files.
+
+The ``-c`` argument specifies the configuration file. This is required,
+but is not currently used by the process.
+
+The ``-p`` argument specifies the PID file. When the ``kea-lfc`` process
+starts, it attempts to determine whether another instance of the process
+is already running by examining the PID file. If one is already running,
+the new process is terminated; if one is not running, Kea writes its PID
+into the PID file.
+
+The other filenames specify where the ``kea-lfc`` process should look
+for input, write its output, and perform its bookkeeping:
+
+-  ``previous`` — when ``kea-lfc`` starts, this is the result of any
+   previous run of ``kea-lfc``. When ``kea-lfc`` finishes, it is the
+   result of this run. If ``kea-lfc`` is interrupted before completing,
+   this file may not exist.
+
+-  ``input`` — before the DHCP server invokes ``kea-lfc``, it moves
+   the current lease file here and then calls ``kea-lfc`` with this file.
+
+-  ``output`` — this is the temporary file where ``kea-lfc`` writes the
+   leases. Once the file has finished writing, it is moved to the
+   ``finish`` file (see below).
+
+-  ``finish`` — this is another temporary file ``kea-lfc`` uses for
+   bookkeeping. When ``kea-lfc`` completes writing the ``output`` file, it
+   moves the contents to the file of this name. After ``kea-lfc`` finishes deleting the
+   other files (``previous`` and ``input``), it moves this file to the ``previous``
+   lease file. By moving the files in this fashion, ``kea-lfc`` and
+   the DHCP server processes can determine the correct file to use even
+   if one of the processes is interrupted before completing its task.
+
+There are several additional arguments, mostly for debugging purposes.
+``-d`` sets the logging level to debug. ``-v`` and ``-V`` print out
+version stamps, with ``-V`` providing a longer form. ``-h`` prints out
+the usage string.